1. Field of the Invention
The present invention involves an x-ray tube with a cathode and an anode and with a catching device to capture backscattered electrons from the anode in the operating state of the x-ray tube. The invention further concerns a method to operate such an x-ray tube.
2. Description of the Prior Art
To generate x-rays with an x-ray tube, electrons are emitted in the operating state of the x-ray tube, which are accelerated in the direction of a positively charged anode through an electric field from a negatively charged cathode. The electrons, which strike the anode in the so-called focus, release at least part of their energy there in the form of x-rays, which reach the outside of the tube through an exit window in the housing of the tube and can be used to generate radiographs.
X-ray tubes can be designed with a single-pole structure, wherein the anode is grounded and the cathode is at a negative potential relative to. Alternatively, in a double-pole structure the housing of the x-ray tube is typically grounded and the cathode and the anode are respectively at negative potential and positive potential relative thereto.
In the operating state of the x-ray tube, some of the electrons that reach the anode rebound from the anode and are then once again accelerated in the direction of the anode by the electric field between the cathode and anode. This process is amplified in double-pole x-ray tubes, in which the anode has a positive potential relative to the grounded housing. These electrons generally do not strike the focal point and result in unwanted extra-focal radiation. Furthermore, the energy of the anode does not correspond to the energy of the desired x-ray radiation. These unwanted effects reduce the quality of the produced x-rays, which, in turn, has a negative effect on the image quality of an x-ray image that is gained with this type of radiation.
To avoid this negative effect, a catching device can be inserted in the x-ray tube between the cathode and anode, which absorbs the electrons that are backscattered by the anode, so as to prevent those electrons from again being accelerated in the direction of the anode.
Catching devices are known that capture backscattered electrons from the anode and that are designed in the form of a shaft or as a specially-formed center part between the cathode and anode. Thermal energy is mainly absorbed through the impact of electrons in these components which are often referred to as backscattered electron acceptors (BSE-catchers). To discharge the resulting heat in an appropriate way, suitable material with thermal conductivity must be used for these components. Backscattered electrons that do not reach the backscattered electron acceptor impact the anode again and consequently raise the temperature of the anode additionally. Thereby the anode generates unwanted extra-focal radiation.